In general, vibration isolating bushings for absorbing or damping vibration are provided in the mounting of a motor vehicle body where the suspension is attached. The vibration isolating bushing generally comprises an outer sleeve or tube, an inner sleeve or tube disposed concentrically with the outer tube, and a rubber-like resilient and elastic member interposed between the outer and inner tubes. The vibration isolating bushings, in order to give a good ride when the vehicle runs on a rough road, need to be made soft. On the other hand, in order to give good steering stability, it is necessary to make the bushings hard or stiff with respect to a lateral force, such as a cornering force. Some of the approaches for fulfilling the aforementioned conflicting needs are described below.
GB 2 033 533 A discloses a rubber-metal bearing for improving a tendency to over-steer during motor vehicle cornering. The bearing has rubber-metal interfaces, (i.e., the inner surface of an outer sleeve and the outer surface of an inner sleeve), at least partially oblique to the axis of the bearing, so that when an axial force is applied to the bearing, the outer and inner sleeves are displaced relative to each other radially along the oblique portions. They thereby produce a radial component-force to reduce or eliminate the over-steer tendency. GB 2033533 A further discloses, a bearing element having the oblique portions only at one end thereof, and teaches connecting two such bearing elements with the respective oblique portions oriented laterally outwardly, in order to make the bearing effective for forces in both axial directions.
Japanese Utility Model Application Laid-Open No. 6-8835 discloses a bushing for use in the link mechanism mounting portion of a suspension. The bushing is constructed so as to have a soft or small spring constant in the longitudinal direction of a motor vehicle and a hard or large spring constant in the lateral, or right and left, direction of the vehicle. More specifically, in this bushing, a bulged hump portion with an inclined surface is formed on an inner tube. An outer tube has a contracted portion at its end comprising a tapered surface opposed to the inclined surface of the inner tube. A rubber-like elastic member is interposed between the inclined surface of the inner tube and the tapered surface of the outer tube. The inclined surface and the tapered surface are paired with each other in the axial direction to thereby produce a radial component from an axial left or right load (input) acting on the bushing. By providing the bushing in the mounting portion of a suspension arm, a soft spring characteristic is provided in the longitudinal direction. Rigidity is high, however, in the right and left or lateral direction, with respect to vibration or load which is input from the road through the vehicle wheels. It is thus possible to satisfy both riding comfort and steering stability requirements.
GB 2 033 533 A, as having described above, teaches connecting two bearing elements so as to provide the oblique portions at respective ends of the connected elements, in order to deal with inputs in both axial directions. Also, in Japanese Utility Model Application Laid-Open No. 6-8835, two component-force generating means, each comprising the paired inclined surface and tapered surface, are provided in the axial direction in a single bushing. In either case, if the rigidity in the axial direction is increased to improve the steering stability, then the spring constant in the longitudinal direction of the vehicle, i.e., in the radial direction of the bushing, will also increase. As a result, the compliance characteristic of the suspension arm in the longitudinal direction, on which the bushing is mounted, is reduced, thereby reducing riding comfort. In addition, the structure of the bushing itself becomes complicated and the manufacturing cost is increased.